Tunable Impedance Matching Circuit

ABSTRACT

The tunable impedance circuit comprises capacitors C 1  and C 2 , an inductor L 1 , and an inductor L 2  magnetically coupled with the inductor L 1 . The control current I control  with variable phase and amplitude from the control circuit  13  flows in the inductor L 2 . The impedance of the inductor L 1  is changed by changing the phase and amplitude of the control current I control . The output impedance is set to an optimum level by setting an effective inductance and an effective quality factor of the tunable impedance circuit  12   a  to be optimum by means of the phase and amplitude of the control current I control  relative to output current I RF  of RF PA  11.

TECHNICAL FIELD

The present invention relates to a tunable impedance matching circuitthat is able to adjust impedance.

BACKGROUND ART

An integrated RF power amplifier (PA) employs an output impedancematching circuit (circuit) to transform the antenna impedance (50Ω ingeneral) into an optimum impedance that promotes, among othercharacteristics, a good performance in terms of maximum output power,linearity, efficiency and stability. This optimum impedance can beviewed as an optimum resistance (R_(opt)) once it is considered that theimpedance matching circuit eliminates the reactive part of the resultingimpedance.

The basis for determining R_(opt) is the load line method described bynon-patent document 1. Once R_(opt) is determined, this value should befine-tuned in order to optimize the performance of the PA in terms of,for instance, efficiency or linearity.

The impedance matching circuit can be integrated or can be placedexternally to the integrated circuit.

FIG. 1 is a schematic diagram of a typical RF power amplifier and animpedance matching circuit.

In FIG. 1, input and output impedance are assumed to be 50 Ohm. Inputimpedance matching circuit 10 is provided at the input of RF poweramplifier (PA) 11 to match the input impedance of 50 Ohms to an optimumimpedance for an input of RF PA 11. Output impedance matching circuit 12is provided at an output of RF PA 11 to match an output impedance of RFPA 11 to the output impedance of 50 Ohms.

The numerous wireless standards available today and the frequency bandsthat are subject to their regulations bring about the need formulti-standard, multi-frequency RF power amplifiers (PAs). Such a devicecan be a wide band PA covering the frequency bands of interest or anarrow band PA whose center frequency can be adjusted when a change inthe band of operation occurs. The latter is the principle behind thefrequency tunable RF power amplifier.

In most power amplifiers of this kind, the tunability issue is alwaysfocused on the output impedance matching circuit design (non-patentdocuments 2, 3 and patent document 1). This is due to the fact that thevalues of the reactances comprising the output impedance matchingcircuit change with frequency and, hence, the load impedance seen by thePA will also vary, thereby forcing the PA to operate under non-optimalconditions at different bands of operation. In previous implementationsof a tunable power amplifier, the output impedance matching circuit ismade tunable by employing one or more variable reactances. However,changing capacitances of capacitors causes a decrease in the Q-value ofthe impedance matching circuit, thereby increasing loss of the impedancematching circuit.

In non-patent document 2, a saturable reactor is used to implement avariable inductor by controlling the permeability of its core through aDC bias current applied into its control winding. The main problem hereis that such a device cannot be integrated. Integration of the RF poweramplifier together with all other parts of the transceiver is desiredfor space saving and, consequently, for the possibility of adding morefunctionality to the device where the transceiver will be used. In thiscase, CMOS is the technology of choice because of its high level ofintegration, low cost and high yield. In non-patent document 3, MEMS areused to switch on and off inductors and capacitors, thereby forming atunable impedance matching circuit. This approach, therefore, relies onthe availability of MEMS, which is not the case for standard processes.In patent document 1, several possibilities of variable reactances areproposed, but varying the two capacitors of a n-circuit is the mainapproach.

Therefore, achieving a frequency tunable RF PA that can be integrated inan IC is important. In order to achieve this, it is vital to solve theproblem of how to construct a tunable impedance matching circuit thatcan be integrated in an IC.

[patent document 1] F. H. Raab, “Electronically tuned power amplifier,”U.S. Pat. No. 7,202,734.

[non-patent document 1] S. C. Cripps, RF Power Amplifiers for WirelessCommunications, 1st ed. Norwood: Artech House, 1999.

[non-patent document 2] F. H. Raab and D. Ruppe, “Frequency-agileclass-D power amplifier,” in 9th International Conference on HF RadioSystems and Techniques, University of Bath, UK, Jun. 23-26, 2003, pp.81-85.

[non-patent document 3] J. L. Bartlett, et al., “Integrated tunable highefficiency power amplifier,”

U.S. Pat. No. 6,232,841, May 15, 2001.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a tunable impedancematching circuit which is easily integrated in an IC.

A tunable impedance matching circuit adjusting the impedance of theinput or output of an external circuit according to the presentinvention, comprises: a first inductor for conducting a current of theexternal circuit; a capacitor unit connected to the first inductor; asecond inductor magnetically coupled with the first inductor, forconducting a control current with a certain phase and amplitude relativeto the current of the external circuit; and a control circuit forapplying the control current to the second inductor and changing theimpedance of the first inductor magnetically coupled with the secondinductor by changing either or both of the phase and amplitude of thecontrol current.

According to the present invention, the inductance of the first inductoris changed by changing the phase and amplitude of the control currentapplied to the second inductor, which is magnetically coupled with thefirst inductor. Because the impedance can be changed only by changingthe current, the configuration is simple and easy to integrate in ICs.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an RF power amplifier.

FIG. 2 is a schematic diagram of a frequency tunable RF power amplifierwith the tunable impedance matching circuit according to the embodimentof the present invention.

FIG. 3 is a schematic diagram of a π impedance matching circuit.

FIG. 4 is a schematic diagram of a tunable inductance based oncoupled-inductors.

FIG. 5 is a schematic diagram of the tunable π impedance matchingcircuit according to the embodiment of the present invention.

FIG. 6 is a layout of the integrated planar-interleaved-squaretransformer according to the embodiment of the present invention.

FIG. 7 is a circuit diagram of the frequency tunable CMOS RF poweramplifier with the tunable impedance matching circuit according to theembodiment of the present invention.

FIG. 8 is a simulation result comparison in terms of output power,efficiency and linearity between the frequency tunable RF poweramplifier according to the present invention and a conventional RF poweramplifier with fixed output impedance.

FIG. 9 is a circuit diagram of the frequency tunable CMOS RF poweramplifier with the possibility of fine tuning the control currentaccording to the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention relate to the tunable impedancematching circuit that is applicable, for example, to the field of RFpower amplifiers to be used in wireless transmitters and transceiversand, more specifically, to techniques that allow these amplifiers tooperate in different frequency bands with optimal performance.

An RF power amplifier is improved by, for example, making it tunable infrequency within specific operating frequency bands by using the tunableimpedance matching circuit of the embodiment. The impedance matchingcircuit of the embodiment employs coupled-inductors. Via the applicationof a control current into one of the windings of thesecoupled-inductors, the impedance matching circuit becomes tunable infrequency, thereby allowing the load impedance of, for example, thepower amplifier to be set to an optimum value at each operating band.

In the application of the embodiment of the present invention, afrequency tunable RF power amplifier employing an output tunableimpedance matching circuit on the basis of integrated planarcoupled-inductors is presented. However, the application of the presentinvention is not limited by the example below. Further, the examplebelow shows that the tunable impedance matching circuit of theembodiment is used for matching output impedance. However, the tunableimpedance matching circuit of the embodiment can be used for matchinginput impedance as well. Further, the circuit used with the tunableimpedance matching circuit of the embodiment can be an arbitrary circuitother than a power amplifier.

FIG. 2 is a schematic diagram of the frequency tunable power amplifierto which the tunable impedance matching circuit of the embodiment isapplied.

In FIG. 2, like numerals are assigned to like components in FIG. 1 andexplanations thereof are omitted.

This amplifier can operate in two or more different bands, for instance,2.4 GHz and 5.2 GHz, to which are allocated the channels for wirelesslocal area network (WLAN) devices. The advantage of this technique istwofold: first, the possibility of adapting the impedance transformationby the use of only one variable reactance and second, the enhancement ofthe quality factor (Q) of the inductor [5], thereby reducing matchingand resistive losses due to its series parasitic resistance.

The above technology is described in the document below. [5] D. R.Pehlke, A. Burstein, and M. F. Chang, “Extremely high-Q tunable inductorfor Si-based RF integrated circuit applications,” in 1997 IEEEInternational Electron Devices Meeting (IEDM'97) Technical Digest,Washington, D.C., Dec. 7-10, 1997, pp. 63-66.

In FIG. 2, the tunable impedance matching circuit of the embodiment isapplied as the output impedance matching circuit 12 a. The tunableimpedance matching circuit 12 a comprises: inductor L1, capacitor C1connected to an input of the inductor L1, capacitor C2 connected to anoutput of the inductor L1, and inductor L2 magnetically coupled to theinductor L1 by a coupling constant k and conducting a control currentI_(control). Control circuit 13 receives an output current of the inputimpedance matching circuit 10 and produces the control currentI_(control) to supply the inductor L2. The impedance of the inductor L1can be changed by changing the amplitude and phase of the controlcurrent, which is an alternating current.

As outlined above, adjusting the load impedance through the use of justone variable reactance is possible by employing a n-matching circuitwith two shunt capacitors and a series inductor.

FIG. 3 shows a n-matching circuit.

One reason for choosing such a circuit is that it provides the choice ofboth the transformation factor (R_(L)>R_(opt)) and its overall qualityfactor (Q₀). The other reason is that by using adequate capacitorvalues, the same optimum transformation factor can be obtained fordifferent frequencies by changing just the value of the inductance (L1).This is the objective of a tunable impedance matching circuit and,hence, tuning the inductor is more effective than tuning the capacitors.For example, for an optimum resistance of 20Ω, an antenna impedanceshould be 50Ω, which results in a transformation factor of 2.5. Thistransformation factor is obtained by choosing C1=5.65 pF and C2=3.8 pF.If the value of the inductor is varied from 0.4 nH at 5.2 GHz to 1.6 nHat 2.4 GHz, the optimum resistance will be 200 at these two frequencies.If the value of the inductor was invariable and equal to 0.4 nH, theresulting resistance due to the transformation at 2.4 GHz would be 1.50.

One important non-ideality of the n-matching circuit is the finitequality factor Q_(u) of the series inductor. This means that a seriesresistor R_(L)s is added to the inductor introducing two mainshortcomings. The first is the power loss due to dissipation in R_(L)sand the second is the power loss due to mismatching in the circuitintroduced by a series resistor placed in the inductor path. Hence, thehigher the quality factor of the inductor the better the performance ofthe power amplifier in terms of maximum output power and efficiency.

Tunable inductors can be built with active inductors [6, 7], MEMSswitches [non-patent document 3], saturable reactors [non-patentdocument 2] and coupledpassive inductors [5, 8-10].

For details, please refer to the documents below.

-   [6] R. Mukhopadhyay, et al., “Frequency-agile CMOS RFICs for    multi-mode RF front-end,” in Proceedings of the 7th European    Conference on Wireless Technology, Amsterdam, Holland, Oct. 11-12,    2004, pp. 9-12.-   [7] J. H. Sinsky and C. R. Westgate, “A new approach to designing    active MMIC tuning elements using second-generation current    conveyors,” IEEE Microwave and Guided Wave Letters, vol. 6, no. 9,    pp. 326-328, September 1996.-   [8] Y.-C. Wu and M. F. Chang, “On-chip high-Q (>3000)    transformer-type spiral inductors,” Electronics Letters, vol. 38,    no. 3, pp. 112-113, Jan. 31, 2002.-   [9] B. Georgescu, et al., “Tunable coupled inductor Q-enhancement    for parallel resonant LC tanks,” IEEE Transactions on Circuits and    Systems Part II: Analog and Digital Signal Processing, vol. 50, no.    10, pp. 750-713, October 2003.-   [10] W. A. Gee and P. E. Allen, “CMOS integrated    transformer-feedback Q-enhanced LC bandpass filter for wireless    receivers,” in Proceedings of the International Symposium on    Circuits and Systems (ISCAS' 2004), vol. 4, Vancouver, Canada, May    23-26, 2004, pp. 253-256.

The only type of tunable inductor that provides the possibility ofquality factor enhancement is the coupled passive inductor. Hence, theoutput impedance matching circuit used in the embodiment employs coupledpassive inductors.

Although only n-matching circuits that have two capacitors are shown inthe figures, a circuit with only one capacitor is also acceptable.

FIG. 4 shows how an inductor can be tuned using mutual inductances.

By applying a control current (I_(control)) through L2 having the sameamplitude as the RF current (I_(RF)) that passed through L1, the totalinductance seen by the RF circuit connected to L1 becomes L_(eq)=L1+M ifthe phase shift (φ) between these two currents is zero. In thisequation, M stands for the mutual inductance between. L1 and L2 andequals: M=k·√(L1·L2) (where k is the coupling factor between L1 and L2).However, if 9 equals 180 degrees, then the total inductance becomesL_(eq)=L1−M. Therefore, varying the phase shift between these twocurrents allows tuning of the total inductance seen by the RF circuitfrom L1−M to L1+M.

Besides the change in the inductance, a resistive part appears in serieswith the impedance seen by the RF circuit. If the amplitude ofI_(control) is r times the amplitude of I_(RF) and if r is varied, theresistive part added by the mutual inductance can attain negative valuesand decrease the effective series parasitic resistance R_(Ls1) _(—)_(eff) of L1, so that R_(Ls1) _(—) _(eff)<R_(Ls1).

For L1=L2=L, R_(Ls1)=R_(Ls2)=R_(Ls), the effective inductance seen bythe RF circuit and its corresponding quality factor can be written as:

$\begin{matrix}{{L_{eff} = {L\left( {1 + {{k \cdot r}\; \cos \; \varphi}} \right)}},{Q_{eff} = \frac{\omega \; {L\left( {1 + {{k \cdot r}\; \cos \; \varphi}} \right)}}{R_{Ls}\left( {1 - {{k \cdot r}\; \sin \; {\varphi \cdot Q_{u}}}} \right)}}} & (1)\end{matrix}$

The equation above shows that L_(eff) has a tuning range that depends onthe amplitude and phase of I_(control) and that its quality factorQ_(eff) can be increased if the term k·r sin φ·Q_(u) is made close tobut less than unity, where Q_(u) is a quality factor when I_(control) isnot applied.

FIG. 5 shows a tunable output n-matching circuit based oncoupled-inductors.

FIG. 6 shows a top-layer planar-interleaved square transformer.

The coupled inductors are implemented with an integrated four-terminalplanar-interleaved transformer. The transformer geometry can be square,octagonal or circular. Its windings can be built with a single top metallayer or with stacked metal layers. The choice of the type oftransformer depends on the current that it must support and on the valueof the inductor, and they will influence the final quality and couplingfactors.

The control circuit is responsible for injecting a current I_(control)in L2 with controlled phase shift (φ) and amplitude ratio (r) inrelation to I_(RF) in the frequency bands in which the tunable RF poweramplifier will be employed.

In FIG. 5, capacitors C1 and C2 and an inductor L1 compose a n-matchingcircuit. The inductor L1 magnetically couples with an inductor L2. Thecontrol current circuit 20 injects the control current I_(control) intothe inductor L2. The control current I_(control) is an alternatingcurrent with variable amplitude and phase.

In FIG. 6, the integrated four-terminal planar-interleaved transformeris constructed by two winding lines. Each winding line has a width w andboth are separated by spaces of width s. The input and output terminalsat (1) are those of the inductor L1 and conduct a current I_(RF) of anRF circuit. The input and output terminals at (2) are those of theinductor L2 and conduct the control current I_(control). The width ofthe coupled-inductor is d_(out).

FIG. 7 shows an example circuit diagram of a tunable RF PA with atunable impedance matching circuit of the embodiment.

In FIG. 7, a concrete circuit configuration of the control circuit isshown. The input impedance matching circuit 21 is a conventionalimpedance matching circuit that has one capacitor C3 and one inductor L3and a bias voltage source. The tunable impedance circuit of theembodiment is applied to an output impedance matching circuit 22 a and22 b. Even though circuits 22 a and 22 b are shown as separatedcircuits, the inductor L1 and inductor L2 of both circuits aremagnetically coupled and therefore both circuits are considered to beone circuit. The control circuit 23 comprises two transistors M2 and M3and a bias voltage source. An RF Choke coil is connected at a drainterminal of the transistor M1.

Transistor M1 is the core of the power amplifier with a fixed inputmatching made with C3 and L3 and a tunable π output impedance matchingcircuit formed by C1, L1 and C2. L1 is magnetically coupled to L2 andthey are both implemented with an integrated planar transformer like theone in FIG. 6. The control current is related to the RF current becausethe control circuit composed of the cascoded transistors M2 and M3 havethe same input signal as that of the PA. The transistor M3 is providedto increase the isolation of a current flowing through the transistorM2. As alternating components of the input I_(RF) to the transistor M1are applied to a gate of the transistor M2, The frequency of the controlcurrent I_(control) becomes equal to the frequency of the input I_(RF)to the transistor M1. The phase shift and amplitude ratio betweencurrents I_(control) and I_(RF) is established by the dimensions of thetransistors and the values of the inductors and capacitors, although theamplitude ratio can be changed by another means. Cascoding in thecontrol circuit and the RC feedback comprising capacitor Cstab and aresistor Rstab in the PA are used to guarantee unconditional stabilityin all frequencies.

In FIG. 7, the phase of the control current I_(control) is fixed so thata quality factor of the tunable impedance matching circuit 22 a and 22 bbecomes optimum. The inductance of the inductor L1 is controlled by theamplitude of the control current I_(control), which can be changed bychanging the voltage of the bias voltage source BIAS2. Here, the optimumquality factor means that a loss of the tunable impedance matchingcircuit becomes minimum.

FIG. 8B shows the simulation result for the output power of the poweramplifier against third-order intermodulation distortion (IMD3) for thetunable PA and for a similar PA with a fixed output impedance matchingcircuit (but with the same L1).

In this example, the tunable power amplifier was designed to operate ina 5.2 GHz band.

In FIG. 8A, the measurement of the power-added efficiency (PAE) for thecircuit is shown.

From this figure, it can be seen that the tunable PA allows a higheroutput power to be delivered (considering a limit of −35 dBc IMD3) inthe 5.2 GHz band with a higher efficiency.

FIG. 9 shows another example circuit diagram of a tunable RF PA with atunable impedance matching circuit of the embodiment.

Once the components of the circuit of FIG. 7 have been dimensioned, thephase relationship between the control and RF currents are fixed. Inorder to allow further flexibility to this circuit so that therelationship between I_(control) and I_(RF) can be adjusted, transistorsM2 and M3 can be split into M2 a, M2 b and M2 c and M3 a, M3 b and M3 c,forming parallel branches a, b and c as shown in a control circuit 23 aof FIG. 9. By connecting the gate of M3 b and M3 c to ground, thesebranches are disabled whereas connecting them to VDD enables them.Enabling and disabling these parallel branches that have different phaseshift characteristics allow the phase and amplitude relationshipsbetween I_(control) and I_(RF) to be varied, thereby allowing theoptimum resistance seen by the power amplifier to be adjusted. Thisflexibility permits the circuit to be fine tuned for proper operation inthe frequency range of interest. To enable and disable these branches,switches S1 and S2 can be used. Bits b0 and b1 disable branches b and cwhen these bits are high and can be implemented as shown in detail inthe box of FIG. 9. When these bits are low, these bits enable branches band c. Transistors M3 and M2 can be split into more branches if moreflexibility is required.

1. A tunable impedance matching circuit adjusting an impedance of aninput or output of an external circuit, comprising: a first inductor forconducting a current of the external circuit; a capacitor unit connectedto the first inductor; a second inductor magnetically coupled with thefirst inductor, for conducting a control current with a certain phaseand amplitude relative to the current of the external circuit; and acontrol circuit for applying the control current to the second inductorand changing the impedance of the first inductor magnetically coupledwith the second inductor by changing either or both of the phase andamplitude of the control current.
 2. The tunable impedance matchingcircuit according to claim 1, wherein the capacitor unit includes afirst capacitor connected at an input of the first inductor; and asecond capacitor connected at an output of the first inductor.
 3. Thetunable impedance matching circuit according to claim 1, wherein thecontrol circuit sets the phase and the amplitude of the control currentso that a quality factor of a circuit comprising the first inductor andthe capacitor unit becomes optimum.
 4. The tunable impedance matchingcircuit according to claim 1, wherein a frequency of the control currentis equal to that of the current of the external circuit.
 5. The tunableimpedance matching circuit according to claim 1 connected at an outputof the external circuit.
 6. The tunable impedance matching circuitaccording to claim 1 connected at an input of the external circuit.
 7. Afrequency tunable amplifier configured with the tunable impedancematching circuit according to claim
 1. 8. A frequency tunable amplifierconnected with the tunable impedance matching circuit according to claim1 at its input.
 9. A frequency tunable amplifier connected with thetunable impedance matching circuit according to claim 1 at its output.